(Not Applicable)
(Not Applicable)
The present invention relates generally to crank assemblies for bicycles, and more particularly to a crank assembly having a radially expandable male crank arm with an exterior mating surface, a female crank arm having with a bore bounded by a cooperatively engageable mating surface for receiving and engaging the male arm, and a connector for expanding the male crank arm and for connecting the male and female arms.
As is known in the art, bicycles are provided with a pedal, or crank, assembly for powering the bicycle. The crank assembly typically includes a pair of crank arms each having a foot pedal mounted outboard at zone end. The opposite ends of the crank arms are connected by a drive shaft, and a drive sprocket is attached to the driveshaft near one of the crank arms to function as the drive gear for a chain drive system. The remainder of the drive shaft is rotatably mounted within a generally cylindrical bottom bracket. The interior of the bottom bracket is normally provided with a pair of annular race bearings within which the shaft rests and is free to rotate. Typically, the bracket also has two thrust bearings to prevent axial movement of the crank arms and a spacer to axially position the race bearings and thrust bearings.
For most crank assembly applications, it is very desirable for the crank arms and shaft to rotate in unison. Any slippage in the connection between the crank arms and shaft reduces the efficiency of the crank assembly. Moreover, pedaling can be uncomfortable if the crank arms periodically slip in and out of engagement with the drive shaft. Thus, the drive shaft and crank arms are often provided with cooperatively engageable splined surfaces to promote a tight connection between the elements. Typically, the ends of the shaft have an exterior splined surface for engaging splined bores disposed in the crank arms.
While there are a number of crank assembly designs in the prior art, the splined connections currently utilized often compromise tightness and durability of the connected elements, or they tend to be expensive to fabricate. For example, some crank assemblies are designed with loosely mated splines in order to reduce manufacturing costs. As a result, the assemblies tend to be less efficient because the gaps between splines allow the crank arms to slip across the surface of the shaft until the splines engage. In addition because splines often have a thin tip and a relatively thick base, the gaps are larger if only the tips engage. Furthermore, large gaps allow the splines to bump and grind against each other to such an extent that pedaling is uncomfortable Bumping and grinding can also cause premature failure of the assembly.
To address these problems, some crank assemblies are fabricated with very small dimensional tolerances to create a tighter fit between the splines. This approach, however, significantly increases manufacturing costs because more sophisticated fabrication procedures and high precision machine tools are required to fabricate the assembly. Still Other crank assemblies have splines sized for an interference fit such that the shaft and crank arms are pounded into an engaged position. This solution provides a snug fit, however, the mechanical strength and stiffness of the splines are altered when the splines are deformed as part of the forced engagement. Additionally, the splines and thrust bearings can be damaged when the shaft and crank arms are forcibly engaged, and the race bearings can be damaged if the pounding force is applied at an angle to the shaft.
In response to these tradeoffs, two piece crank assemblies have been developed in which one crank arm has an externally splined male shaft, and the opposite arm has an internally splined female shaft sleeve for receiving and engaging the shaft. This design eliminates the need to fabricate a separate shaft, however, two piece assemblies have heretofore been less durable. For instance, the female sleeve is usually split longitudinally, that is, it has a C-shaped profile when viewed from the end. With this configuration, the sleeve wraps around the shaft and one or more pinch bolts close the sleeve into an engaged position with the shaft. This design initially provides a tight fit between the shaft and sleeve, however, rigorous pedaling creates shaft stresses and bending moments which can deform the sleeve and pull the sleeve apart along the split. As a result, the tightness of the fit tends to deteriorate with usage.
In view of the above considerations, a primary object of the present invention is to provide a two piece crank assembly in which a splined male shaft is radially expanded into an engaged position with an internally splined female shaft sleeve to minimize gaps between the splines.
Another object of the present invention is to provide a two piece crank assembly in which the splined working surfaces are radially supported and fixed in an engaged position to maintain tight engagement over the operating life of the crank assembly.
Yet another object of the present invention is to provide a crank assembly which is relatively inexpensive to manufacture.
These and other objects of the present invention will become apparent throughout the description thereof which now follows.
The present invention is an expansion engagement crank assembly for a bicycle. The assembly includes a female crank arm, a male crank arm, and a connector for connecting the female arm and the male arm. The female arm has a shaft sleeve attached at one end, and the sleeve has an inboard end, an outboard end and a bore originating at the inboard end. The bore is bounded by a mating surface and a fastener surface.
The male arm has a shaft attached at one end, and the shaft has a radially expandable inboard end and an outboard end. The inboard end of the shaft is sized for insertion within the bore of the sleeve sand has a complimentary mating exterior surface for engaging the mating surface bounding the bore. The shaft additionally has an interior coupling surface defining a passage extending through the shaft from the inboard end to the outboard end.
The connector is sized for placement within the bore of the sleeve and the passage of the shaft. The connector is also shaped for engaging the fastener surface bounding the bore and for engaging the coupling surface defining the passage to connect the male arm with the female arm. The connector is additionally shaped to radially expand the mating surface of the shaft into an engaged position with the mating surface of the sleeve when the connector connects the male arm and female arm.
The crank assembly here taught thus provides an effective mechanical joint to minimize slippage of the crank arms when a bicyclist pedals the assembly. In addition, the connector radially supports the shaft and shaft sleeve so they are maintained in an engaged position. Furthermore, the assembly is relatively inexpensive to manufacture because the shaft and shaft sleeve can be fabricated with greater dimensional tolerances and a tight fit between the shaft and shaft sleeve can be achieved by expansion engagement.